Hello fellow non-londoner
Okay this is going to be such a long reply so I apologise in advance.
There are plenty of UK people who apply that are not from London, so don't worry you wouldn't be the only one! Don't get me wrong, there are plenty who are from London who choose to go to UCL/other London unis over others because they love the city so much, but there's still a mix. I'd say on my course, we have about 15/60 from the UK, 6 of which are from London and the rest from all over the UK (2 from Wales, 1 from Devon and the rest more North). So I really wouldn't worry about that, you are very likely to meet people who are from similar areas to you too.
In terms of where graduates go, I think that really depends on your degree (so I'm a Cell Biologist so most of my friends and I will be going into further education rather than applying for jobs off the bat) and also depends on their own career aspirations. But you really shouldn't see the distance as a bad thing - it gives you an excuse to travel and see different places. One of the great things about having friends from all over Europe is that the holidays (when a pandemic isn't strife) are fantastic
Also, 3/4 years in Uni/London might give you a different perspective over what you want to do/where you want to be. You might end up wanting to stay in London for a career too - or even outside of the UK. In any case, whatever uni you go to, everyone will inevitably follow their own career path and go where that takes them. In the meantime, just enjoy being a student
With my friend, she said that she doesn't think it was a coincidence that EVERYONE in her flat was french (but we can't be sure). Honestly she would have preferred to have a bit more variety, but she had a good time all the same.
I met the majority of my friends on my degree course. Most of the degrees at UCL have their own society (so we have a 'Biological Sciences Society') which hold their own freshers week events just for course students. It's such a blast, and so much fun. Almost every student will tell you that the general freshers week is the most overrated thing, most of the time because you have the same conversation with 100+ people 'What degree are you from? Where are you staying? Where are you from originally' and then you move on. What makes freshers events organised by societies so good is that they actually organise ice-breakers and stuff so you end up having such a good time and ACTUALLY start talking to people. Also, there's no better bonding activity than writing an assignment the night before in the library with 15 of your coursemates. The panic, the caffeine and the hysteria that hits about an hour before submission is UNMATCHED. Mind you, this also depends a lot on your cohort - so my degree has around 60 people, so we all know each other very well (most of us moved in with each other in second/third year). The bigger degree courses its a little harder to get acquainted with everybody.
Also bear in mind that this was my own personal experience. Whether you are a coursemate person or flatmate person is quite split across the uni. Some people bond strongly with their flatmates (that's all luck of the draw!). Also, you meet a lot of people with similar interests as you when you join clubs and societies. I met one of my closest friends in Netball club (the social one - not the teams one lmao). So I highly encourage you to join as many as you can in first year, its a lot of fun and you meet so many great people.
AND FINALLY your last question. Yes, you pay a LOT more for rent than you would in any other city, that's for damn sure. London rent is expensive, and you need to be aware of this. BUT it's completely manageable. In first year, I was paying £145 a week for my uni accom. In second year, I moved into a flat that was an 8 minute walk from uni (a godsend) but my rent shot up to £180 a week. The closer you live to uni, the more rent you pay (I was completely aware of this, but my flatmates and I preferred to live close than further out). You can live 2/3 tube stops/ 30-40 minute walk away from uni and pay instead £150, its a choice you and your flatmates have to make yourselves. A couple of friends did just that and they found it annoying that they couldn't just pop home during lunch breaks/etc (so once they were on campus they tended to stay the whole day) but wouldn't change a thing because they saved a lot of money. I also had a job during uni (the student union website has a jobshop, where they advertise jobs specifically for students) which meant that for me, rent was completely manageable. You also save a lot of money during first year which can be saved as rent money for the second etc. In terms of space, you would be very lucky (or just rich) to get a place with a living room, unless you move further out. My flat in second year was quite big (we had huge rooms) and we had a kitchen with a dining table which was our version of a common room, which we used a lot. Again, it all depends on how much you're willing to spend!
Sorry for the very long reply, I hope I've covered all your questions tho!